Direct current (DC) motors are caused to rotate upon application of a DC voltage. Generally, the angular frequency of the motor increases as the applied DC voltage increases. Thus, the angular frequency of the motor can be controlled through the applied DC voltage. DC motors are used for a wide variety of applications. For instance, smart phones may use eccentric rotational mass (ERM) DC motors to cause a vibration for haptic feedback to the user of the phone (e.g., vibration upon receipt of a text message). Numerous other applications exist for DC motors including medical tools, fan control, pump control and toothbrushes.
Some applications for DC motors benefit from tight control of the motor's angular frequency (i.e., speed). Unfortunately, due to manufacturing variations, DC motors of a common type (e.g., same part number) do not all exhibit the same angular frequency for a given DC voltage. Friction, for example, is not a commonly well-controlled parameter and thus affects angular frequency differently from actuator to actuator. Such manufacturing variations may impair the use of DC motors in various applications.